The multicarrier transmission method performs parallel data transmission by dividing the data into a plurality of subcarriers. Since a symbol period can be elongated as compared to single carrier transmission, transmission deterioration caused by multipath can be reduced. Also, since OFDM, which is an efficient means to realize the multicarrier transmission, performs signal transmission by means of a plurality of orthogonal subcarriers, high frequency use efficiency and high-speed transmission can be achieved.
FIGS. 1A-1B are diagram illustrating a schematic configuration of a transmitter and a receiver in which the multicarrier transmission method is employed. FIG. 1A is a transmitter, and FIG. 1B is a receiver. In FIG. 1A, an input data signal is converted into a plurality of parallel symbols by means of a serial/parallel converter (S/P) 1, and by means of subcarrier modulators 2, a plurality of subcarrier signals which are complex modulated into I/Q components are generated. Also, an inverse fast Fourier transform unit (IFFT) 3 performs inverse fast Fourier transform of each subcarrier signal. To the transformed output signal, a guard interval is added by a guard interval (GI) inserter 4, and further, after being frequency converted into an RF signal (not illustrated), the signal is transmitted from an antenna.
In the receiver, an inverse operation to the transmitter is carried out. Specifically, first, a timing detector 5 detects the symbol timing (which may also be referred to as synchronous timing), that is, the position of the guard interval. The detection of the symbol timing is achieved by the known self-correlation method, by which the guard interval position is detected by the detection of self-correlation with a received signal being time-shifted by the length of an effective symbol length.
Based on the detected symbol timing, a guard interval eliminator 6 eliminates the guard interval from the received input signal, and a fast Fourier transform unit (FFT) 7 performs fast Fourier transform of the received signal, using the symbol timing as FFT start timing, so as to regenerate the plurality of subcarrier signals. Subcarrier demodulators 8 then demodulate the subcarrier signals, which are then restored to a serial signal by means of a parallel/serial converter (P/S) 9. Thus, the original data is output and demodulation processing is performed by a reception processing unit 20.
The guard interval (GI) is formed by the addition of the latter part of the symbol to the top of the symbol by duplication. By the addition of GI, it becomes possible to suppress transmission deterioration caused by intersymbol interference etc. produced in a multipath propagation path.
The multicarrier transmission method (particularly OFDM) is already put into practical use in terrestrial digital television, wireless LAN, etc. Also, in recent years, application to a wireless communication system such as mobile telephony is proceeding. At that time, a mobile station such as a mobile telephone terminal may perform handover processing to switch over base stations to communicate with. To facilitate the switchover operation to the base station for communication by the handover, the coincidence of predetermined synchronous timing, which is to be a criterion for communication, may be made between each base station.
As a means for coinciding the synchronous timing between the base stations, for example, a GPS system using GPS (Global Positioning System) and a master-slave synchronization system to subordinate a plurality of base stations by the disposition of a highly stable reference oscillator in a base station are already known.
In the GPS system, each GPS receiver is installed on the basis of a cell covered by each base station, and a reference clock being output from the GPS receiver is used as a synchronous timing signal of each base station. For this purpose, facilities such as an external antenna and a feeder line are required for the base station to obtain the reference clock from the GPS receiver.
Also, according to the master-slave system, it is necessary to introduce a reference oscillator to generate highly stable clock signals, and to provide a master-slave function from the base station having the reference oscillator installed thereon.
Further, when a mobile station starts communication with a second base station after performing handover from a first base station through which communication is in progress, there is a known method that, prior to the start of handover, the mobile station receives a monitoring slot from the second base station, so as to establish synchronization using a known pilot symbol assigned to the monitoring slot (Patent document 1). Additionally, in a wireless communication system employing a multicarrier transmission method, as a technique to simplify handover processing, there is known a technique that a subcarrier set, including a plurality of designated subcarriers, is set in each base station, and the plurality of subcarriers in the subcarrier set being set in each base station are subcarriers different from a plurality of subcarriers in a subcarrier set being set in each adjacent base station, so as to prevent the use of neighboring subcarriers within each subcarrier set (Patent document 2).    [Patent document 1] Japanese Laid-open Patent Publication No. 2002-300628.    [Patent document 2] Japanese Laid-open Patent Publication No. 2006-333452.